Use of Water Bottles And Ice Cream

I think its just best to get everything in the store for now since you might need them in the future. At least that's what I think

the general priority for event shops is something like
1. Welfare servant items
2. CEs
3. Ascension mats
4. Fous
5. Everything else

week after event end you going to lose every event related items except craft event, shiki ascension material (Sharp Knife). So its recommend to spend it all in shop - water bottles , ice cream and black cat figurine don't have any other uses in game